Beirut: Two senior Islamic State leaders were killed in an air strike in northeastern Syria on Monday, the Syrian Observatory for Human Rights reported.

The Observatory, which gathers its information from an activist network on the ground, identified the leaders as an Iraqi, Abu Osama al-Iraqi, and a Syrian named Amer al-Rafdan.

The Observatory said the air strike was believed to have been carried out by the U.S.-led coalition that is targeting Islamic State in Syria and Iraq, though Rami Abdulrahman, who runs the Observatory, said he could not be sure.

The Syrian army and militia fighting alongside it are mounting a separate campaign against Islamic State in the region that borders Turkey to the north and Iraq to the east.

The ultra-hardline Islamic State is fighting in Hasaka province against both pro-government forces and Syrian Kurdish fighters who have received air support from the U.S.-led coalition.

U.S. President Barack Obama said last week the coalition was intensifying its campaign in northern Syria. The Kurdish YPG militia have made a series of advances against Islamic State in the north, including the capture of the town of Tel Abyad at the Turkish border.

Abdulrahman said al-Iraqi had the title of "governor" of the province Islamic State had declared in northeastern Syria. Rafdan had previously served as Islamic State`s governor of Deir al-Zor province, he said.

Islamic State last year declared a cross-border "caliphate" claiming to rule over all Muslims. The U.S.-led alliance has ruled out the idea of cooperation with Syrian President Bashar al-Assad, seeing him as part of the problem.